For many internal combustion engine applications, such as for powering heavy trucks, it is desirable to operate the engine in a braking mode. This approach involves convening the engine into a compressor by cutting off the fuel flow and opening the exhaust valve for each cylinder near the end of the compression stroke.
An early technique for accomplishing the braking effect is disclosed in U.S. Pat. No. 3,220,392 to Cummins, wherein a slave hydraulic piston located over an exhaust valve opens the exhaust valve near the end of the compression stroke of an engine piston with which the exhaust valve is associated. To place the engine into braking mode, three-way solenoids are energized which cause pressurized lubricating oil to flow through a control valve, creating a hydraulic link between a master piston and a slave piston. The master piston is displaced inward by an engine element (such as a fuel injector actuating mechanism) periodically in timed relationship with the compression stroke of the engine which in turn actuates a slave piston through hydraulic force to open the exhaust valves. The compression brake system as originally disclosed in the '392 patent has evolved in many aspects, including improvements on the control valves (see U.S. Pat. Nos. 5,386,809 to Reedy et al. and 4,996,957 to Meistrick) and the piston actuation assembly (see U.S. Pat. No. 4,475,500 to Bostelman). A typical modern compression braking system found in the prior art is shown in FIG. 8, where the exhaust valves are normally operated during the engine's power mode by an exhaust rocker lever. To operate the engine in a braking mode, a control valve separates the braking system into a high pressure circuit and a low pressure circuit using a check valve which prevents flow of high pressure fluid back into the low pressure supply circuit, thereby allowing the formation of a hydraulic link in the high pressure circuit. A three-way solenoid valve, positioned upstream of the control valve, controls the flow of low pressure fluid to the control valve, and thus, controls the beginning and end of the braking mode.
Various problems have been discovered with conventional compression braking systems. First, an unnecessarily long inherent time delay exists between the actuation of the three-way solenoid valve and the onset of the braking mode. This time delay is in part due to the positioning of the solenoid valve a spaced distance from the control valve creating longer than desired fluid passages and thus response time. Also, unnecessarily long fluid passages between the master and slave pistons, that is, the high pressure circuit, disadvantageously increases the compressed fluid volume and thus the response time. In addition, in conventional compression braking systems, the braking system is a bolt-on accessory that fits above the overhead. In such systems, in order to provide space for mounting the braking system, a spacer is positioned between the cylinder head and the valve cover which is bolted to the spacer. This arrangement adds unnecessary height, weight, and costs to the engine. Many of the above-noted problems result from viewing the braking systems as an accessory to the engine rather than as part of the engine itself.
One possible solution is to integrate components of the braking system with the rest of the engine components. One attempt at integrating parts of the compression braking system is found in U.S. Pat. No. 3,367,312 to Jonsson, which discloses an engine braking system including a rocker arm having a plunger, or slave piston, positioned in a cylinder integrally formed in one end of the rocker arm wherein the plunger can be locked in an outer position by hydraulic pressure to permit braking system operation. Jonsson also discloses a spring for biasing the plunger outward from the cylinder into continuous contact with the exhaust valve to permit the cam-actuated rocker lever to operate the exhaust valve in both the power and braking modes. In addition, a control valve is used to control the flow of pressurized fluid to the rocker arm cylinder so as to permit selective switching between braking operation and normal power operation. However, the control valve unit is positioned separately from the rocker arm assembly, resulting in unnecessarily long fluid delivery passages and a longer response time. This also leads to an unnecessarily large amount of oil that must be compressed before activation of the braking system can occur, resulting in less control over the timing of the compression braking. Furthermore, the control valve is used to control the flow of fluid to a predetermined set of cylinders in the engine thereby undesirably preventing individual engine cylinders or different groups of engine cylinders from being selectively operated in the braking mode. Moreover, the control valve is a manually operated rotary type valve requiring actuation by the driver often resulting in unreliable and inefficient braking operation. Also, rotary valves are subject to undesirable fluid leakage between the rotary valve member and its associated cylindrical bore. The Jonsson braking system also relies on a single cam lobe and rocker arm assembly to move the rocker arms during both normal power and braking operations. However, this arrangement disadvantageously restricts the system's ability to provide exhaust valve operation which is independent from normal valve operation as determined by the associated normal cam profile.
U.S. Pat. No. 3,332,405 to Haviland discloses a compression braking system wherein a control valve unit, for enabling the formation of a hydraulic link, is mounted in a cavity formed in a rocker arm that operates the exhaust valves during the braking mode. Separate cam lobes are used for normal power operation and braking operation. However, a single rocker arm is used to actuate the exhaust valves during both normal and braking modes possibly causing the braking cam lobe profile design, and therefore the braking system operation, to be at least partially dependent on, or influenced by, the design of the cam lobe used for operating the exhaust valve during normal engine operation. In addition, Haviland appears to use a single solenoid for controlling compression braking for all of the cylinders, which permits either none or all of the cylinders to be used for compression braking at any one point, and therefore permits only one level of compression braking power. This restriction results in very little freedom in the operation of the compression braking system. Furthermore, the reference discloses a solenoid valve unit, for controlling the flow of fluid to the control valve, which is housed separately from the control valve unit and the rocker lever, resulting in the need for extra dedicated space in the engine; thus adding to the size and weight of the engine. In addition, the control valve as disclosed in Haviland, and conventional control valves generally, use one or more springs to bias the control valve element. However, these springs are subject to repeated reciprocal motion and excessive stress causing spring failure and thus significant reliability problems, resulting in malfunctioning of the control valve and the compression braking system.
U.S. Pat. No. 4,251,051 to Quenneville discloses a solenoid valve assembly having an inlet communicating with a supply of fluid, and one or more outlet passages communicating with respective loads requiring intermittent fluid supply and a drain passage. A respective ball valve is positioned between the inlet and each outlet and spring biased to block flow between the supply and outlet passage while opening the drain passage. An armature and pin are actuated to move the ball valve so to connect the supply to the outlet, and close the drain passage. However, when the valve assembly in the actuated position permits supply flow to the outlet passage, it does not prevent the return flow of fluid from the outlet passage into the supply passage and therefore could not permit the formation of a hydraulic link between different pressurized circuits as required by a control valve during compression braking system operation.
U.S. Pat. No. 3,921,666 to Leiber discloses a solenoid-operated valve assembly having first and second closure members and a spring positioned therebetween for biasing the members toward respective closed positions blocking fluid flow through respective fluid passages. A solenoid device operates the valve such that when the solenoid is not energized, fluid flows between a first and second connection, while a third connection is closed off by the second closure member. When the solenoid is slightly energized, the first closure member cuts off the communication between the first and second connection, while the second closure member keeps the third connection closed. A higher energization of the solenoid forces the second closure member to open, creating a path for fluid between the second and third connections. However, during higher energization of the valve permitting flow between a supply and load, the second closure member is not operable to block return flow from the load. As a result, this valve does not provide for a integral check valve for allowing fluid to enter a hydraulic circuit without allowing fluid to flow in the opposite direction; that is, from the hydraulic circuit to the supply. Therefore, this valve assembly could not be used in a braking system to create a high pressure hydraulic link between an exhaust valve and a cam lobe while permitting intermittent filling of the high pressure circuit forming the link. In addition, the intermediate stage of slightly energizing the solenoid to achieve the desired flow patterns as discussed above is incompatible with the flow characteristics desired in a compression braking system.
In addition, U.S. Pat. Nos. 2,944,565 to Dahl, 4,460,015 to Burt et al. and 4,844,119 to Martinic disclose other three-way structures for controlling fluid flow. However, these valves suffer from the same shortcomings and problems discussed hereinabove with respect to Haviland, Quenneville, and Leiber.
The timing of the opening and closing of the exhaust valves plays a major role in determining the efficiency and effectiveness of the compression braking system. Many conventional braking systems rely on existing engine components to determine the timing of the exhaust valve opening and closing during compression braking. For example, the braking system shown in U.S. Pat. No. 4,592,319 to Meistrick, utilizes a fuel injector actuation mechanism, such as a cam lobe and push rod, that is normally actuated near the end of the compression stroke. However, reliance on existing cam lobes and other actuators used to actuate other engine components severely limits the spectrum of possibilities for timing the operation of the exhaust valve, thereby precluding optimization of the braking system. U.S. Pat. Nos. 4,572,114 to Sickler and 4,898,206 to Meistrick et al. disclose similar compression braking systems suffering from the same disadvantages as the system disclosed in the '319 reference.
U.S. Pat. No. 5,146,890 to Gobert et al. discloses a method and device for compression braking wherein a dedicated cam lobe operates an exhaust valve during the braking mode. However, the cam lobe operates a dedicated exhaust valve used only during the braking mode. As a result, this design is unnecessarily expensive due to costs relating to the additional exhaust valve assembly and the redesign of the cylinder head to include the additional exhaust port and exhaust valve access passage. Also, this design undesirably creates additional packaging considerations in positioning the exhaust valve in the cylinder head.
Consequently, there is a need for a simple, compact, yet effective braking system which is capable of minimizing the size and weight of the associated engine while ensuring optimum operation of the compression braking system.